In a network environment, an initiating computer (i.e., a client) sends a message to another computer (i.e., a server) requesting that the server perform some action. The World Wide Web employs this technique. A web browser (i.e., the client) requests web pages from a web server (i.e., the server).
This technique has been widely used because it is easy to model and because it clearly distributes responsibilities between the client and the server. The server can perform only operations which the server exposes to the network. Upon an exception or undesirable results, the server contacts the client for instructions. Often, it would be desirable to resolve the exception or undesirable results at the server without further contact with the client.